There has been described in my U.S. Pat. No. 3,806,903 and in the art cited therein a class of digital signal translating and/or memory devices which utilize movable cylindrical magnetic domains sustainable in uniaxially anisotropic ferromagnetic crystal platelets. Some of the properties of such cylindrical magnetic domains were also described in an article which appeared at page 78 of the June, 1971 issue of "Scientific American," which article was entitled, "Magnetic Bubbles," by Andrew H. Bobeck and H. E. D. Scoville.
From the above sources it will be noted that two of the most commonly used types of crystals for these devices are the orthoferrites and the garnets. The invention herein described is particularly suitable for but not restricted to the orthoferrites. From the above sources it will also be noted that, as stated on page 85 of the Bobeck article, there are two general methods for controlling the motion of such bubbles. The first employs conductors in which flowing currents generate the driving fields. This method is called conductor access. The second, which is called field access, involves immersing the entire wafer in either a pulsating or rotating magnetic field that acts on the bubble by means of carefully placed patterns of magnetic material that concentrate the field. The present invention is directed to devices utilizing conductor access techniques with orthoferrite crystals.
Where the field access technique is used, it is desirable to have a crystal which inherently has as high a mobility or as low a coercivity as is possible in order that the driving field in which the device is immersed may be as efficient as possible. The carefully placed patterns of magnetic material that concentrate the field in such devices are used to define the paths of travel or the locations of bubbles in the crystal in a manner well illustrated by the photographs on page 86 of the Bobeck article. As may be seen therein, the T-bar patterns are considerably larger than the diameter of a bubble and even the "angel fish" patterns are composed of dots, each of which are nearly as large as a bubble. In this type of field access device these "Permalloy" patterns perform the function of determining bubble location which function is performed by the magnetic fields of the electrical conductors in a conductor access device. The inherent increase of coercivity resulting from these patterns is an undesired detriment in these rotating field access devices and the pattern does not extend over any significant portion of the device, but is confined to the minimum area necessary to determine guideways or bubble locations.
As is pointed out on page 88 of the Bobeck article, the bubbles which are moved through these positions determined as described above may conventionally be detected by electromagnetic induction, the Hall effect, direct optical sensing, or magneto-resistance techniques. None of these techniques utilizes the unmodified guiding "Permalloy" patterns to also facilitate bubble detection.
The class of device to which the present invention is directed is a conductor access type as described in my issued U.S. Pat. No. 3,806,903. These conductor access devices use ferromagnetic crystal platelets formed of one of the orthoferrites. Such crystals have sufficient intrinsic coercivity themselves to make the conductor access techniques feasible. However, if one attempts to increase the efficiency of such devices by smoothing out the intrinsic coercivity in the crystal with heat treatment or the like, it is found that a considerable lack of homogeneity still remains and that it is not possible to smooth out small irregularities in bulk and surface properties of the crystal.
It is a purpose of the present invention to provide a means for uniformly controlling the localized coercivity over an extended area of the crystal platelet which means simultaneously affords an improved method of detecting the movement of the magnetic domains.